Introduction

THERMAL BARRIER COATINGS (TBCs) are applied to metallic components of advanced heat engines to reduce the metal temperature, increase the environmental resistance and life of the component, and in some cases reduce noxious exhaust emissions. This thin coating usually consists of a metallic bond coat applied to the metal component, followed by a layer of magnesia- or yttria-stabilized zirconia (YSZ). If the TBC is on nickel- or cobalt-base superalloy components intended for high-temperature operation, it is advisable to heat treat the TBC to achieve bond coat densification and diffusion bonding to the substrate. The selection of the TBC system involves many considerations, principally knowledge of the intended service conditions, temperature reduction expectations, component life estimates, cost, and in some cases the external surface finish requirements. It is implicit that a successful TBC will also survive the rigors of service without spalling or eroding. This article discusses the various tests applied to the TBC system, and to the zirconia layer separately, to establish thermal design properties and thermomechanical and environmental stability.

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